Surface plasmons are electromagnetic surface waves confined to a metal-dielectric interface by coupling to the free electron plasma in metals. Due to their evanescent nature, surface plasmon waves are not limited by the diffraction limit, and can provide confinement of light on scales much smaller than the free-space wavelength. The possibility of subwavelength confinement and control of optical fields has generated intense interest in the rapidly developing field of plasmonics and plasmonic devices.
Because the surface plasmons allow light to be concentrated in nanometer-scale volumes, surface plasmons have many applications including bio-sensing, data storage, photonics, and solar cells, for example. Traditionally, plasmonics has been utilized in spectroscopic techniques such as surface enhanced raman spectroscopy, where intense local fields allow the detection of single molecules. More recently, the ability to couple light to surface plasmons has been examined for reducing optical circuit elements such as waveguides, filters, switches for example, to sizes much smaller than the optical wavelength. As a result, the field of plasmonics has arisen to study how man-made metallic structures can control the generation, propagation, and manipulation of surface plasmons.
One observation that has spurred this development is the enhanced optical transmission through thin metal films that are perforated with an array of sub-wavelength holes. Because the holes are periodically spaced, diffraction can excite surface plasmons when the film is irradiated. The surface plasmons can then transmit energy through the holes and re-radiate on the opposite side of the film. This leads to (i) transmission much higher than would be expected for sub-wavelength holes and (ii) electric fields that are highly concentrated within the holes. More recently, the same effects were observed when a single hole in a metal film was surrounded by circular concentric grooves patterned on both sides of the film. See Lezec, H. J.; Degiron, A.; Devaux, E.; Linke, R. A.; Martin-Moreno, L.; Garcia-Vidal, F. J. and Ebbesen, T. W.; “Beaming light from a subwavelength aperture,” Science 297, 820 (2002). In this case, the surface plasmons interact with the quasiperiodicity of the structure to couple to the far field. Furthermore, even with a sub-wavelength hole, the transmitted beam is highly directional, a property useful for many applications.
Since the discovery of these effects and others, many researchers have begun incorporating these phenomena into plasmonic devices. Typically an optical signal is utilized to create the surface plasmons. Once formed, they can then be manipulated on the device before re-radiating as an optical output. However, in many cases it may be more convenient to have a non-optical source for surface plasmons on the device.
In many devices, surface plasmons are created when a metal film, such as silver or gold, is illuminated on an exposed surface of the metal film. Although this excitation process is theoretically forbidden when the surface is perfectly flat, surface patterning on a sub-optical length scale (micrometers or nanometers, for example) allows incident light to generate plasmons in the metal film. Patterning also provides a means to manipulate such plasmons once the plasmons are created. Because the plasmons exist very close to the surface of the metal film, the plasmons are sensitive to surface inhomogeneities, which can cause one or more of absorption, scattering, and limited propagation, for example.
Several methods are known for fabricating patterned metal surfaces with features on a sub-micrometer or nanometer length scale. For example, in one method a metal film is deposited on a surface of a substrate such as by using thermal evaporation or sputtering. After depositions the metal film is patterned to have sub-optical length scale features by conventional lithography steps such as by using photolithography or e-beam lithography, for example. Alternatively, after deposition, focused ion beam etching is used to pattern the metal film. Using either approach, sub-micrometer features can be formed in the metal film.
Methods for forming patterned metal surfaces having sub-optical length scale features, such as the deposition methods described above, have several shortcomings. For example, one limitation relates to surface roughness. As noted above, plasmons are sensitive to surface inhomogeneities, thus it is desirable for the surfaces of the metal film to be smooth and free from undesirable surface inhomogeneities. Such smooth surfaces are desired both in the patterned regions and the unpatterned regions. This is because surface plasmons can be scattered during propagation across a metal surface by such roughness leading to potential losses and undesirable performance.
Using the methods described above, the surfaces of the metal film in the patterned regions are typically rough for several reasons. Generally, roughness of the surfaces of the metal film in the patterned regions is undesirable. After deposition, metallic films are typically polycrystalline. That is, the metal includes many individual small grains of crystal that are randomly oriented in space. Patterning by conventional techniques such as reactive ion etching or focused ion beam etching exposes each of the individual metallic grains differently due to anisotropic etching in the material. This leads to roughness of the surfaces in the patterned regions after the patterning operation.
The surfaces of the unpatterned regions are also typically rough for several reasons. Generally, roughness of the surfaces of the unpatterned regions is also undesirable. In particular, the surfaces of the metal film in the unpatterned regions are usually rough due to the presence of the grains themselves. In other words, deposited metallic films have inherent surface roughness that is present even before the metal films are patterned. In any event, such surface roughness, whether in the patterned regions, unpatterned regions, or both, often causes losses and undesirable performance.
Another limitation of known methods for forming patterned metal surfaces, such as the deposition methods described above, is that such methods are not generally applicable to all metals. In particular, known methods for forming patterned metal surfaces are difficult to apply to high melting temperature refractory metals, such as tungsten, molybdenum, tantalum, and niobium. This difficulty arises because two major classes of patterning techniques, (i) imprinting, embossing, or molding, and (ii) lithography (patterning a thin film with a sacrificial pattern followed by etching), are not easily applicable to refractory metals. The first class of techniques (imprinting, embossing, or molding) is commonly used in the fabrication of compact discs and diffraction gratings. Patterning a refractory metal with these techniques is difficult because it would require heating the metal to an extremely high temperature where the metal softens. Further, these materials have inherent hardness and toughness, which causes wear on the molds. Typical molds used for embossing, such as silicon hard molds, would also not survive the high operating temperatures required to pattern these materials. The second class of fabrication techniques, lithography (etching with a mask), suffers from contamination and cost issues as well as limitations on the roughness and scale of the resultant pattern. In addition, with such a technique, etching recipes must be determined for each refractory material, some of which are not available. Both classes of techniques are also difficult to apply to metals with good wear resistance such as titanium. The difficulty arises not only from hardness but also because they typically etch slowly, which limits the use of sacrificial masks in lithography.
Additional known methods for fabricating patterned metal surfaces include nanoimprinting and nanomolding. Although nanoimprinting and nanomolding can pattern metals on the proper length scales, undesirable surface roughness is usually present in metal surfaces formed by nanoimprinting and nanomolding. In a typical process, a patterned polymeric mold is filled with metal to form a replica. This produces undesirable surface roughness because metals do not easily wet the surfaces of the polymeric mold. Moreover, an additional shortcoming of nanoimprinting and nanomolding is that the polymeric mold needs to be etched away from the metal film to release the metal film. Accordingly, each mold can only be used once to produce a single metal film.
Another technique that can be used to fabricate smooth metal surfaces is generally referred to as template stripping. Template stripping, utilizes the poor adhesion and good wettability of noble metals on solids such as mica, glass, and silicon. In a typical template stripping process, a freshly cleaved mica surface is coated with a film of gold. The exposed surface of gold is then attached to another substrate with an epoxy adhesive. When the mica and substrate are separated the gold adheres to the substrate by the epoxy and is released by the mica surface. The resulting gold surface typically has a roughness related to the roughness of the mica surface.
The above-described template stripping method, however, is limited to use with generally flat surfaces and has not successfully been utilized with surfaces including three-dimensional features such as those typically found on patterned metal films. This is because the addition of three-dimensional features generally increases the area of mica in contact with gold. As this contact area increases it becomes more difficult to separate the gold film from the mica surface. Moreover, such three-dimensional features can interfere with separation of the gold from the three-dimensional surface features. Where a patterned metal having three-dimensional features is desired, the above nanoimprinting and nanomolding techniques are typically used wherein the mold is etched away from the metal film.